This invention relates to improvement in ammonia synthesis trains.
In current practice almost all synthetic ammonia is produced with the use of hydrocarbon feedstocks to furnish the source of the hydrogen required in the catalytic synthesis step, the necessary nitrogen being derived from air.
To minimize compression costs, the hydrogen is produced from the hydrocarbon feedstocks by reforming or partial oxidation at elevated pressures of, e.g., from 100 to 1500 psig (pounds per square inch gage). Reforming or partial oxidation produces a mixture of hydrogen and carbon monoxide which is then treated to convert the carbon monoxide to hydrogen and CO.sub.2 by the so-called shift reaction after which the CO.sub.2 is removed prior to ammonia synthesis. Depending upon the process route chosen (i.e., reforming or partial oxidation), the required nitrogen is added to the hydrogen either prior to carbon monoxide shift and CO.sub.2 removal or following these steps. Using reforming, the nitrogen is introduced by adding air during the reforming operation, whereas in partial oxidation trains, the nitrogen is added to the hydrogen following CO.sub.2 removal and prior to compression to ammonia synthesis pressures.
In such ammonia synthesis trains, the final production cost of the synthetic ammonia is almost entirely a function of the capital costs of the plant and the cost of the hydrocarbon feed. Both of these factors are importantly affected by the capital cost and the efficiency of the systems used for shift conversion of carbon monoxide to hydrogen and the removal of carbon dioxide. This invention is concerned with improvements in the arrangement of the carbon monoxide shift system and the CO.sub.2 removal system and the manner in which these systems are integrated so as to provide an integrated system which is low in capital cost and high in thermal efficiency and which minimizes process gas losses in the ammonia synthesis train.
Currently, the most commonly used ammonia synthesis process employs gaseous or light liquid hydrocarbons as feedstocks and steam reforming processing. The hydrocarbon feed together with steam is treated over a reforming catalyst at temperatures generally ranging from 1000.degree. to 1500.degree. F and pressures generally ranging from 100 to 600 psig. The nitrogen in the proper ratio required in the ammonia synthesis reaction is added in the form of air in a secondary reforming step. The carbon monoxide-hydrogen mixture produced by reforming is then treated in a series of so-called shift conversion steps to convert the carbon monoxide to hydrogen and CO.sub.2. The process stream, now consisting essentially of hydrogen, nitrogen and CO.sub.2, is treated for the removal of CO.sub.2 following which small residual amounts of CO and CO.sub.2 are removed usually by a methanation step which converts the residual CO and CO.sub.2 into methane by reaction with hydrogen contained in the gas over a methanation catalyst. The process gas is then cooled and compressed to ammonia synthesis pressures of, e.g., 2000 to 8000 psig.
After contact with the ammonia synthesis catalyst, ammonia is recovered from the process gas and unconverted synthesis gas is recycled in the so-called ammonia recycle loop to be retreated over the ammonia synthesis catalyst. Because of this recycle operation, inert materials (mainly methane and argon) tend to build up in the recycle loop. In order to maintain the concentration of these inerts at a reasonable level in the recycle loop, it is necessary to continuously purge a portion of the recycled gas from the loop at a rate which will keep the inerts at a constant tolerable level. The purge gas consists mainly of hydrogen and nitrogen together with some unrecovered ammonia and these inerts, and has little value except as a waste fuel gas. The loss of hydrogen and ammonia in the recycle loop purge gas can represent a substantial loss of the total potential yield of ammonia.
In ammonia trains based on the steam reforming of gaseous or light liquid hydrocarbon feedstocks, the most common procedure is to use two stages of carbon monoxide shift conversion in order to reduce the residual carbon monoxide in the synthesis gas to a relatively low level. In the first stage, the bulk of the carbon monoxide is converted in a so-called high temperature shift converter normally employing an iron oxide catalyst promoted with small amounts of another metal oxide such as chromium oxide at temperatures generally in the range of from 600.degree. to 1000.degree. F. The high temperature employed in the first shift conversion stage favors rapid reaction and thus minimizes the amount of catalyst required. On the other hand, the shift equilibrium constant, K.sub.p for the reversible shift reaction: EQU CO + H.sub.2 O.revreaction.CO.sub.2 + H.sub.2
is less favorable at higher temperatures. The equilibrium constant K.sub.p for the above shift reaction may be expressed as: ##EQU1## where the parentheses indicate the partial pressure of the component designated within the parentheses at equilibrium. At high temperatures, the values for K.sub.p are relatively low, reflecting relatively lower degrees of conversion of carbon monoxide. Typically residual carbon monoxide content in high temperature shift effluent may be in the range of from 2.5 to 3.5% by volume (anhydrous basis).
Since it is uneconomic to leave relatively high concentrations of unconverted carbon monoxide in the synthesis gas (e.g., 2.5-3.5%), a second shift conversion stage is employed using a so-called low temperature shift catalyst, typically a reduced copper catalyst promoted with zinc oxide and generally operating at temperatures in the range of from 350.degree. to about 550.degree. F. In the second stage of conversion, the shift equilibrium K.sub.p is much more favorable and typically, the carbon monoxide can be reduced to residual levels of the order of 0.2-0.5% by volume (anhydrous basis) leaving the low temperature shift conversion stage.
The process gas, after having thus been treated successively in high temperature and low temperature shift conversion stages, now contains typically from 16 to about 25% CO.sub.2 by volume (anhydrous basis). CO.sub.2 removal is then carried out, usually by scrubbing with an alkaline liquid scrubbing agent such as an aqueous solution of potassium carbonate or of an ethanolamine to produce a gas containing generally less than 0.2% CO.sub.2. The process gas now containing typically 0.3% carbon monoxide and .2% or less of carbon dioxide is then further treated over a methanation catalyst to convert the residual CO and CO.sub.2 to methane by the following series of reactions: EQU CO + 3H.sub.2 .fwdarw.CH.sub.4 + H.sub. 2 O EQU co.sub.2 + 4h.sub.2 .fwdarw.ch.sub.4 + 2h.sub.2 o
note that for each mole of CO converted to methane, three moles of hydrogen are consumed and for every mole of CO.sub.2 methanated, four moles of hydrogen are consumed. The hydrogen for the methanation reaction is of course supplied by the hydrogen in the process steam. For example, in a typical case where 0.3% CO and 0.2% CO.sub.2 is methanated, there is a loss of approximately 2.3% of the total hydrogen in the synthesis gas. Even more significant than this substantial hydrogen loss in the methanation step, the conversion of the residual amounts of CO and CO.sub.2 to methane substantially increases the inerts contents of the synthesis gas and very substantially raises the purge losses of hydrogen and ammonia in the recycle loop of the ammonia synthesis reactor. In the typical case discussed above, the methanation of the residual CO and CO.sub.2 would introduce approximately 0.5% methane into the ammonia synthesis gas. This increased inerts content in a typical ammonia synthesis loop would result in hydrogen loss in the purge gas equal to approximately 5% of the total hydrogen. In most typical ammonia trains in current operation, losses of ammonia production due to these losses, i.e. loss of hydrogen in the methanation step, and loss of hydrogen and ammonia in the purge step, may run up to 10% or more of total ammonia production, equivalent in a large plant to losses of several million dollars a year in ammonia product.
There have been numerous suggestions for improving the efficiency of ammonia synthesis trains by introducing a CO.sub.2 removal step between shift conversion stages in order to reduce the concentration of CO.sub.2 entering the final shift conversion stage to a low level in order to achieve a high degree of conversion of the CO in the final shift. By reducing the concentration of CO.sub.2 in the final shift conversion stage, more complete conversion of the CO is of course favored because of the reversible nature of the shift reaction. See, for example, U.S. Pat. No. 2,487,981 to Reed, U.S. Pat. No. 3,382,045 to Habermehl et al., and U.S. Pat. No. 3,577,221 to Smith et al. While a small number of commercial plants have employed a CO.sub.2 removal step between stages of shift conversion, only minor advantages have been obtained in contrast to the increased cost and complexity of the CO.sub.2 removal system, the necessary additional heat exchange equipment required, and the additional heat losses incurred as the process stream is alternately cooled and heated between successive shift conversion and CO.sub.2 removal stages. As a consequence, the great majority of ammonia trains do not use between-shift CO.sub.2 removal systems.